title: Comparison of ReaxFF, DFTB, and DFT for phenolic pyrolysis. 1. Molecular dynamics simulations.
abstract: A systematic comparison of atomistic modeling methods including density functional theory (DFT), the self-consistent charge density-functional tight-binding (SCC-DFTB), and ReaxFF is presented for simulating the initial stages of phenolic polymer pyrolysis. A phenolic polymer system is simulated for several hundred picoseconds within a temperature range of 2500 to 3500 K. The time evolution of major pyrolysis products including small-molecule species and char is examined. Two temperature zones are observed which demark cross-linking versus fragmentation. The dominant chemical products for all methods are similar, but the yields for each product differ. At 3500 K, DFTB overestimates CO production (300-400%) and underestimates free H (~30%) and small C(m)H(n)O molecules (~70%) compared with DFT. At 3500 K, ReaxFF underestimates free H (~60%) and fused carbon rings (~70%) relative to DFT. Heterocyclic oxygen-containing five- and six-membered carbon rings are observed at 2500 K. Formation mechanisms for H2O, CO, and char are discussed. Additional calculations using a semiclassical method for incorporating quantum nuclear energies of molecules were also performed. These results suggest that chemical equilibrium can be affected by quantum nuclear effects at temperatures of 2500 K and below. Pyrolysis reaction mechanisms and energetics are examined in detail in a companion manuscript.
authors.affiliation: Department of Materials Science and Engineering, Stanford University , Stanford, California 94305, United States.
authors.forename: Tingting
authors.initials: T
authors.lastname: Qi
authors.forename: Charles W
authors.initials: CW
authors.lastname: Bauschlicher
authors.forename: John W
authors.initials: JW
authors.lastname: Lawson
authors.forename: Tapan G
authors.initials: TG
authors.lastname: Desai
authors.forename: Evan J
authors.initials: EJ
authors.lastname: Reed